1. Field of the Invention
The present invention relates to an electroacoustic transducer which converts an electric signal into sound by electromagnetic conversion and a method of winding a coil therein.
2. Description of the Related Art
FIG. 6 shows an internal structure of an average electroacoustic transducer of prior art. The electroacoustic transducer comprises the components of a housing 102, a yoke 104, a pole 106, a coil 108, a magnet 110, a diaphragm 112 etc. A magnetic piece 114 is attached to the central portion of upper surface of the diaphragm 112 as a means for increasing the substantial oscillating mass thereof, and the housing 102 forms a resonance chamber 116 at the upper side of the diaphragm 112 to which the magnetic piece 114 is attached. A sound emitting cylinder 118 is formed in the housing 102 as a means for emitting resonance sound generated in the resonance chamber 116 to the outside. The sound emitting cylinder 118 comprises a sound emitting hole 120 therein for allowing the resonance chamber 116 to be open to the atmosphere.
The yoke 104 is provided at an opening formed on the rear side of the housing 102, the pole 106 constituting a magnetic core is attached to the center of the yoke 104 at a base portion 107 thereof by way of press fit etc. and the coil 108 is wound around the pole 106. The cylindrical magnet 110 is provided around the coil 108 and the diaphragm 112 is provided at the upper surface side of the magnet 110. The diaphragm 112 formed of a plate of magnetic material is held on the magnet 110 by the magnetic force thereof. There is a gap 122 between the lower surface of the diaphragm 112 and the end surface of the pole 106 forming a space for permitting the diaphragm 112 to vibrate therein.
In such an electroacoustic transducer, the magnet 110, the yoke 104, the pole 106, the gap 122, the diaphragm 112 and the magnetic piece 114 form a closed magnetic path. The magnet 110 applies a bias magnetic field to the diaphragm 112. The coil 108 comprises terminals, not shown, to which an electric signal to be converted into sound is applied. When the electric signal energizes the coil 108, an alternating magnetic field is generated about the pole 106 of magnetic core to be applied to the diaphragm 112 so as to vibrate the same. The vibration of the diaphragm 112 vibrates air in the resonance chamber 116 to generate resonance sound therein, which is emitted to the outside through the sound emitting hole 120. The level and frequency of this sound depend on the inputted electric signal, and it is known that the acoustic characteristic of the electroacoustic transducer largely influences the characteristic of the generated sound as another element.
The magnetic field generated about the pole 106 depends on the number of turns of the coil 108. That is, although increasing the number of turns of the coil 108 is necessary to generate a stronger magnetic field, the electroacoustic transducer is required to be made small, so that there is naturally a limitation in increasing the number of turns of the coil 108.
In a conventional electroacoustic transducer as illustrated in FIG. 6, a side surface 124 of the coil 108 at the tip end side of the pole 106 has been made flat. It has been a common form of the coil 108 in case the same is wound around the pole 106 directly or by way of a bobbin.
On the other hand, for example, "an electroacoustic transducer" disclosed in Japanese Utility Model Laid-Open Publication No. 2-120998 teaches winding a coil around a pole to form a flat surface conforming to the tip end surface of the pole and then retracting the side surface gradually toward the outer periphery thereof to form a conical side surface. This method expands an effective space for the coil, but unreasonable in that the coil must be made small in height since the amplitude of vibration is maximum at the center of the diaphragm.
In case of the electroacoustic transducer, the automation of manufacturing is requested for reducing the manufacturing cost and meeting the increase of demand. In case of conventional electroacoustic transducers, components are individually machined to be assembled manually thereafter. Therefore, continued processes of forming components and automation of assembling the electroacoustic transducer have been tried for reducing the manufacturing cost.
Moreover, although the electroacoustic transducer is requested to be miniaturized for use in a portable telephone etc., miniaturization to the extreme causes the deterioration of vibration characteristic of the decrease of magnetic force generated by the coil, so that it has to meet a contradictory request of miniaturization without the deterioration of acoustic performance or the decrease of magnetic force generated by the coil.